Problematic code:
for s in "$(mycommand)"; do echo "$s"; done
Correct code:
The correct code depends on your intention. Let's say you're in a directory with the files file.png and My cat.png, and you want to loop over a command that outputs (or variable that contains):

    hello world
    My *.png
Loop over each line without globbing (hello world, My *.png)

    mycommand | while IFS= read -r s; do echo "$s"; done
Loop over each word with globbing (hello, world, My, file.png, My cat.png):
# relies on the fact that IFS by default contains space-tab-linefeed
for s in $(mycommand); do echo "$s"; done
Loop over each line with globbing (hello world, My cat.png)
# explicitly set IFS to contain only a line feed

    IFS='
    '
    for s in $(mycommand); do echo "$s"; done
Rationale:
You get this warning because you have a loop that will only ever run exactly one iteration. Since you have a loop, you clearly expect it to run more than once. You just have to decide how it should be split up.

